Large ducted fans are commonly used in the cooling towers of electric utilities to remove heat from the cooling water of heat exchangers. These fans are made up of four to twelve blades which range from 5 to 20 feet (1.5 to 6.1 meters) in length. A standard twelve foot (3.7 meter) blade employing the NACA 632-615 airfoil from root to tip has been the most commonly used blade in cooling tower applications. This airfoil has a chord thickness of 15%, and it is designed for an operating lift coefficient of 0.6 with a low-drag range that extends from a lift coefficient of 0.4 to 0.8. It was initially designed in the early 1940's for use in general aviation and has been in use over the past 50 years. As a result, certain prior art design objectives have evolved over the course of these years.
The moist environment found in cooling tower applications causes soiling and leading edge corrosion of the fan blades. These conditions result in a roughness effect that reduces the overall aerodynamic performance and efficiency of the fan. Thus, one design objective has been to improve the aerodynamic performance and reduce the sensitivity to roughness under these conditions, while operating at the maximum lift coefficient (c1, max), in order to lower the power requirements on the system.
Optimization of the blade geometry and duct designs for large ducted fans would minimize the power that is required for a given thrust level or an associated pressure increase. One way to increase the thrust-to-power ratio (T/P) is to reduce the drag coefficient of the blade's airfoils to cause a reduction in the power required to drive the blade. A maximum power reduction of 5% has been associated with zero profile drag for the blade. However, because zero drag cannot be accomplished, a realistic drag loss objective could result in a 2% power reduction.
The tip airfoil should be thin enough to provide low drag, but should also provide a maximum lift-to-drag ratio (l/d) at high values of lift coefficient to minimize blade solidity. In the hub region, blade-element performance predictions have indicated the presence of low blade angles of attack. As a result, the root airfoil should produce a high lift coefficient at zero angle of attack. Designing new airfoils, having a minimal sensitivity to roughness, is therefore desirable should the blade operate in a stalled condition. Stalled conditions are usually caused either by an unsteady inflow or the low air density which is encountered when operating the fans at high temperatures.
One of the most desirable design objectives for good performance with a ducted fan is to satisfy the free-vortex flow condition. A fan satisfying the free vortex flow condition has the product of induced inplane swirl velocity and radius being constant along the span of the blade. This causes the radial pressure gradient to balance the centrifugal forces on the fluid and eliminates spanwise (radial) flow and losses due to turbulent mixing. The free-vortex condition dictates the product of local blade chord and lift coefficient. The product of these two parameters results in the necessary radial loading and the resulting fan thrust. The airfoil lift coefficient is derived for known inlet conditions of advance ratio, blade pitch, and twist angle. Therefore, either a value of lift coefficient or chord must be chosen and the other is calculated to provide an optimum combination along the span.
Near the tip region high values of lift coefficient increase the T/P ratio of the fan. Therefore, the operating lift coefficient is selected to coincide with the airfoil's best l/d ratio and the product of the lift coefficient and chord are selected in order to design the fan to a specific thrust, for a given diameter and number of blades.
Near the hub the blade requires high twist to achieve a positive angle of attack. Unlike the tip, it becomes undesirable to twist the blade root toward c1, max and the solidity or blade chord must also increase to satisfy the free-vortex condition. Special care must be taken in the design so that the solidity does not become excessive resulting in adverse “cascade” losses.
In view of the foregoing considerations there is an apparent need to satisfy the foregoing design objectives by providing an airfoil useful in a large tapered/twisted fan blade application but which has an improved aerodynamic performance over the prior art. Improvements in the aerodynamic characteristics are needed to provide an advanced airfoil having a maximum lift coefficient (c1, max) that is designed to be largely insensitive to the effects of roughness and allows a lower solidity blade with lower cascade flow losses.